1. Field of the Invention
The present invention relates to a system for controlling driving of a stepping motor, which is arranged to stop erroneous operation of a motor drive circuit employing a constant-current drive circuit.
2. Related Background Art
Recently, applicant proposed in the U.S. application Ser. No. 08/447,753 filed on May 23, 1995 a drive control system for a stepping motor that permits rotation at a high and constant velocity and at high accuracy while avoiding waste power dissipation.
In this application, a PWM unit in an MPU generates a PWM signal associated with a changing duty ratio, a low-pass filter converts a stepped voltage level represented by the changing duty ratio of the PWM signal into a smooth and almost continuous reference voltage, a coil in the stepping motor is excited by a current set for the reference voltage, and the current thus set is maintained by a constant-current drive circuit.
The motor drive circuit in the drive control system for the stepping motor in the above-mentioned application employs a constant-current drive circuit of a type in which the current can be arbitrarily set. This constant-current drive circuit is arranged to maintain the current supplied to the coil in the stepping motor at a constant designated value and to permit the designated current value designated to be arbitrarily set. In further detail, the current supplied to the coil in the motor is set in such a manner that a voltage associated with the current value is detected and the current is maintained constant by controlling an output from a comparator, depending upon whether the voltage thus detected is higher or lower than the reference voltage from the low-pass filter.
In the case of the constant-current drive circuit in the above-mentioned application, the reference voltage from the low-pass filer needs to be set to zero in order to decrease the current supplied to the coil in the motor down to zero. However, even though the reference voltage is set to 0 V, the voltage cannot perfectly reach 0 V [GND (ground) level] because of the output offset of a reference voltage setting circuit, etc. Namely, the PWM unit in the MPU outputs the PWM signal as a logic signal of either a high level or a low level, and the low level signal of the logic signal rarely takes 0 V in practice, but has a voltage of not more than 0.6 V. Because of it, even when the reference voltage is set to 0 V, the reference voltage from the low-pass filter, input into the comparator in the constant-current drive circuit, is not actually 0 V, but a voltage of not more than 0.6 V. This possibly causes the comparator to fail to operate normally. Even though the reference voltage actually reaches the GND level, the comparator could fail to operate normally because of the influence of noise.
The constant-current drive circuit in the previous application is composed of discrete components, and thus, the entire circuit becomes expensive and the area it occupies on a printed board is large. These factors hinder efforts to reduce the cost and size of an apparatus employing a constant-current drive circuit. Moreover, normally, the power is always supplied to the circuit and, in order to lower the dissipation power of the circuit when not driving the motor, separate circuit and control terminals for controlling it in a standby state are provided, which makes the circuitry complicated and which requires extra control lines.